In conventional assemblies of substrates and electronic components, electrical circuitries carried by the substrates have terminals for connection to electrical terminal leads of the components. For this purpose, a solder is used which has a lead constituent. These lead containing solders are known to be largely reliable for forming permanent electrically conductive solder joints between terminals and electrical conductive leads. However, lead, it is now realized, is an undesirable material to be used in industry. When used in solder, it is not a captive constituent of the solder material and is known to be environmentally unfriendly and is toxic. Conceivably it is dangerous for use both by the manufacturing worker and by the consumer. Lead presents health hazards in manufacturing, disposal, and use of assemblies, and problems arise in the safe disposal of scrap materials containing lead.
Various assemblies have been made in which leadless solder has been used. However, problems are found during manufacturing where lead is not a component of solder as resultant solder joints are found to be unreliable as separation and cracking occurs.
In Great Britain Patent 2,158,459 Tallis teaches a leadless solder and method for forming a fluid-tight, soldered joint between a pair of tubular members of copper or copper alloy in the plumbing industry, the solder comprising 0.3%-0.7% copper by weight, the balance being tin and impurities. However, considerations such as electrical conductivity, miniature component sizes and thermal cycling over wide temperature ranges are quite different in the electronics industry. For example, oxides must be completely eliminated from electronic solder joints because they affect signal transmission and weaken the metallic integrity of the joints, thus causing cracking. These cracks create capacitances at high frequencies, and thus techniques employed by Tallis are inapplicable to assembling electrical components to substrates.
In U.S. Pat. No. 4,778,733 Lubrano also teaches a low toxicity, corrosion resistant solder for use in plumbing. It comprises a tin content of 92% to 99%, a copper content of 0.7% to 6%, and a silver content of 0.05% to 3%. As silver is an expensive precious metal, its use should be avoided to keep the solder from being prohibitively expensive.
"Whiskers" are hairline cracks that form on component leads as a result of a build up of internal stresses as due to plating. When using tin, which has a melting temperature of 232.degree. C., as a plating metal, it was found that the standard tin/lead solder having a percentage weight composition of 63%/37% respectively and a melting temperature of 183.degree. C., never raised the temperature of the tin finishes on the components high enough to relax the stresses, thus providing a candidate for component failure.